Polycarbonate resins are useful materials valued for their physical and optical properties. Methods for the preparation of polycarbonate resins include interfacial processes and melt processes. In interfacial processes, as described, for example, in U.S. Pat. No. 4,360,659 to Sikdar, a bisphenol is reacted with phosgene in the presence of solvents. In melt processes, as described, for example, in U.S. Pat. No. 3,153,008 to Fox, a bisphenol is reacted with a diaryl carbonate. Melt processes are advantageous because they avoid the use of phosgene and solvents.
Use of a melt process for polycarbonate synthesis requires an industrially efficient process for producing diaryl carbonates. There are several known processes for producing diaryl carbonates. One example of such a process is described by U.S. Pat. No. 4,182,726 to Illuminati et al. In this process, diaryl carbonates are produced by reacting dialkyl carbonates with a phenol in the presence of a catalyst such as AlCl3.
U.S. Pat. No. 4,182,726 also demonstrates that diaryl carbonates can be reacted together with dihydric phenols to produce polycarbonates (see Scheme I, below).

A preferred process for making dialkyl carbonates is illustrated in Scheme II, below, and described, for example, in U.S. Pat. Nos. 5,527,943 to Rivetti et al.; and 4,218,391 and 4,318,862 to Romano et al.

As can be seen in Scheme (II) the reaction produces water as a by-product. Also, hydrochloric acid (HCl) may be continuously added to the reaction mixture to maintain a desired molar ratio of chloride to copper. Therefore, HCl, CuCl catalyst, and water are typically found in the stream exiting the reactor vessel. Hydrochloric acid and copper chlorides are very corrosive in the presence of water, so equipment made from corrosion-resistant materials, such as glass-lined or tantalum vessels, must be used in the reaction section of a chemical plant making dialkyl carbonates by this process. However, even corrosion resistant materials can suffer deterioration over time.
Accordingly, there is therefore a need for a dialkyl carbonate process that reduces the amount of corrosion.